1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly to a vertical type power semiconductor device such as a power MOSFET (Metal Oxide Silicon Field Effect Transistor), an SI (Static Induction) thyristor, and an IGBT (Insulated Gate Bipolar Transistor) power transistor, and to a method of manufacturing such a vertical type power semiconductor device.
2. Description of Related Art
The above mentioned vertical type power semiconductor devices have been widely used as a power semiconductor device and has been described in the following documents.
1. Junichi Nishizawa: "High Power Vertical Type Junction FET having Triode Characteristics", Nikkei Electronics, Sep. 27, 1971, pp. 50-61 PA0 2. J. Nishizawa, T. Terasaki and J. Shibata: "Field-Effect Transistor versus Analog Transistor (Static Induction Transistor)", IEEE Trans. on Electron Device, ED-22(4), 185 (1975) PA0 3. J. Nishizawa and K. Nakamura: Physiquee Appliquee, T13, 725 (1978) PA0 4. J. Nishizawa and Y. Otsubo: Tech. Dig. 1980 IEDM, 658 (1980) PA0 5. J. Nishizawa, T. Ohmi, T. Sha and K. Mototani: Technological Report of the Electron and Communication Society, ED81-84 (1981) PA0 6. M. Ishidoh, et al: "Advanced High Frequency GTO", Proc. ISPSD, 189 (1988) PA0 7. B. J. Baliga, et al: "The Evolution of Power Device Technology", IEEE Trans. on Electron Device, ED-31, 1570 (1984) PA0 8. M. Amato, et al: "Comparison of Lateral and Vertical DMOS Specific On-resistance", IEDM Tech. Dig., 736 (1985) PA0 9. B. J. Baliga: "Modern Power Device", John Wiley Sons, 350 (1987) PA0 10. H. Mitlehner, et al: "A Novel 8 kV Light-Trigger Thyristor with Over Voltage Self Protection", Proc. ISPSD, 289 (1990)
In the semiconductor device of the type mentioned above, a current passing between main electrodes can be controlled by controlling a gate voltage, so that such semiconductor device may be advantageously used in a power transducer and switching power source as a high speed switching element. Particularly, in a vertical type semiconductor device in which a current flows vertically within a substrate, it is possible to control a very large current.
In such a semiconductor device, a silicon substrate has been generally used as a semiconductor substrate. In the silicon substrate there are formed channels and currents pass through these channels and semiconductor regions adjacent to the channels.
In the power semiconductor device, a channel and adjoining semiconductor regions have certain resistance even in a conductive state (generally termed as an on-resistance). Therefore, heat is generated during the conduction state. In the case of a silicon substrate, a threshold temperature below which the device operates safely is about one hundred and several tens degree in Celsius's temperature scale. Therefore, when a large current flows, a semiconductor breakdown voltage of device might be decreased and an efficiency of the device might be reduced by a loss due to heat. Therefore, it has been proposed to decrease the on-resistance as much as possible. For instance, in order to reduce a resistance of the channel, it has been proposed to reduce a length of the channel. However, this solution is limited by a fine working, so that the channel resistance could not be decreased to a desired value.
It has been further proposed to make a semiconductor substrate of a material having a higher threshold temperature for safety operation. Such a solution has been described in the following reference.
11. M. Bhatnagar and B. J. Baliga:
"ANALYSIS OF SILICON CARBIDE POWER DEVICE PERFORMANCE"
Proc. 3rd International Symposium on Power Semiconductor Devices and Ics, 176-180 (1991)
In this reference, it is proposed to use a silicon carbide substrate in a vertical type semiconductor device. As compared with the silicon substrate, the silicon carbide substrate has several advantages such as a wide band gap, high electron mobility, high saturation drift velocity, high thermal conduction and high breakdown voltage. Such properties show that the silicon carbide substrate is particularly suitable for power semiconductor devices. However, it is practically very difficult to manufacture a semiconductor device comprising a silicon carbide substrate. Therefore, in the above reference, characteristics of SiC power MOS FET are derived by theoretical calculations. This simulation indicates that the on-resistance of the SiC power MOS FET is smaller than that of the Si power MOS FET by about 2000 times and a breakdown voltage of the SiC power MOS FET amounts to about 5000 V.
As stated above, it is practically difficult to form the silicon carbide substrate having good properties. Thus, it is not possible to realize the power semiconductor device having excellent characteristics even if the theoretical calculation indicates such excellent characteristics. Furthermore, while a silicon carbide substrate could be manufactured, the cost of the silicon carbide substrate is higher than the silicon substrate. Since the silicon carbide substrate can be treated or machined only with difficulty, a manufacturing cost of the silicon carbide substrate would become very expensive.